In SX Microcontrollers, SX/B Compiler and SX-Key Tool, Beau Schwabe wrote:

Electronegativity,

Last night it occurred to me...  The 2mA LEDs that I have are about normal brightness (what I would expect on an appliance) ... 
...BUT If I use a super bright LED and supply a minimal amount of current,  it it's still going to be VERY bright.  Doing just that on a
red LED with a forward voltage of the LED measuring 1.65V (<-- Diode check on a DMM) and a fresh 9V battery measuring 9.23V ,
I used a current limiting resistance of 2K (two 1K's in series).  With only 3.79mA of current to the LED and lights out, I could see
the projection of the LED light on the ceiling.  The LED was aiming straight up from my work bench.

I_to_LED = (Vsupply - LEDforward) / CurrentLimitResistor

I_to_LED = (9.23V - 1.65V) / 2000Ohms

I_to_LED = 7.58V / 2000Ohms

I_to_LED = 3.79mA

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